Texas Sheet Cake, Gluten Free

Instructions

1. 1

   Prep the pan

   Grease an 18x13-inch half sheet pan generously, including the corners and inside walls. Hold off on heating the oven — the batter rests before it bakes, so you'll start preheating partway through.

2. 2

   Whisk the dry ingredients

   In a large bowl, whisk together the gluten-free flour blend, xanthan gum (skip it if your blend already contains xanthan or guar gum), granulated sugar, and salt. Hold the baking soda back for now — it gets added later, right before baking. Whisk for a good 30 seconds; gluten-free blends clump and the xanthan needs to be evenly dispersed.

3. 3

   Boil the butter, water, and cocoa

   In a medium saucepan over medium heat, combine the butter, water, and cocoa. Stir until the butter melts, bring to a full rolling boil for about 30 seconds, then remove from the heat.

4. 4

   Combine and rest

   Pour the hot cocoa mixture over the dry ingredients and stir until smooth. Cover the bowl and let it rest at room temperature for 25 minutes. This is the step that matters most with gluten-free flour: rice flour and starches absorb liquid slowly, and the rest is what turns a gritty batter into a smooth one. Skip it and the cake will taste sandy.

5. 5

   Heat the oven

   About 10 minutes into the rest, set a rack in the middle of the oven and heat to 350°F.

6. 6

   Add the buttermilk, eggs, and soda

   Stir the baking soda into the buttermilk, then whisk in the beaten eggs and vanilla. Pour into the rested batter and mix just until uniform, about 30 seconds. Adding the soda now rather than earlier means its lift isn't wasted during the rest. The batter will be thin and pourable.

7. 7

   Bake

   Pour into the prepared pan, tilt to fill the corners, and bake 20 to 24 minutes. Gluten-free batter runs a couple of minutes behind, and the toothpick test is less reliable here — look for a matte surface that springs back under a light press and edges that have just begun to pull from the pan.

8. 8

   Start the frosting before the cake is out

   When the cake has about 6 minutes left, combine the butter, cocoa, and milk in a medium saucepan over medium heat. Stir constantly until the butter melts and it reaches a simmer, then remove from the heat. Whisk in the powdered sugar a cup at a time, then the vanilla and salt, until glossy and smooth.

9. 9

   Frost immediately

   As soon as the cake comes out, pour the warm frosting over it in wide ribbons and spread quickly with an offset spatula, using as few passes as possible. Scatter the pecans over the top right away so they set into the frosting.

10. 10

    Cool fully before cutting

    Let the cake rest at room temperature for at least 90 minutes — a little longer than the wheat version, since gluten-free crumb is fragile while warm and firms up noticeably as it cools. Once the frosting looks dull and set, cut into 24 squares with a thin knife, wiping the blade between cuts.

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Baker's Tips

• 1.

  Toast the pecans on a dry sheet pan at 350°F for 6 to 8 minutes before chopping. It takes almost no time and makes an enormous difference against all that sweetness.

• 2.

  No buttermilk? Stir 1 1/2 teaspoons of lemon juice or white vinegar into 1/2 cup of whole milk and let it sit for 10 minutes. Sour cream thinned with a splash of milk works too.

• 3.

  Have the frosting ingredients measured and the saucepan ready before the cake goes in. The window between pulling the cake and pouring the frosting should be under a minute.

• 4.

  Dutch-process cocoa gives a darker color and smoother, less acidic flavor; natural cocoa gives a sharper, more old-fashioned chocolate tang. Both work, so use what you have.

• 5.

  If your frosting stiffens before you finish spreading, whisk in warm milk a teaspoon at a time until it loosens.

• 6.

  Baking in a 9x13-inch pan instead will give you a much thicker cake — add 10 to 15 minutes and check the center carefully.

• 7.

  This genuinely improves overnight. The crumb settles and the frosting fuses into the top, so make it a day ahead when you can.